In an all-wheel drive motor vehicle wherein a second axle (usually the rear axle) is driven via a slip-controlled clutch, the problem exists that the rear wheels tend to over-brake. This problem arises because of the connection to the front wheels and results in the loss of road adherence and vehicle swerving. These conditions are especially critical when lock-up braking occurs or when an ABS (anti-lock brake system) is used.
For this reason, a free-wheeling coupling (sometimes referred to as an overrunning clutch) is normally used so that no controllable separation clutch is needed. A free-wheeling coupling has, however, the disadvantage that it must be locked when all-wheel drive is needed also in reverse. To overcome this disadvantage, a double-acting free-wheeling coupling which makes the transfer of drive forces possible in forward and reverse travel may be used. A double-acting free-wheeling coupling has the disadvantage, however, that in forward travel as well as in reverse travel uncoupling occurs when the vehicle is in thrust mode, and this may result in dangerous situations on icy mountain roads or off the road. (In this description, the "towing mode" refers to the situation when the engine drives the vehicle and torque flows from the engine to the axle, whether the vehicle is in forward or reverse travel. "Thrust mode" refers to the situation wherein the engine brakes the vehicle and torque flows from the axle to the engine.)
For this reason, double-acting clamping element couplings have been proposed, in which elements sensing rotational speed differences ensure coupling in both directions of torque flow and where centrifugal elements prevent torque flow reversal at higher speeds (e.g., the German patent applications DE 42 01 375, 42 02 152 and 42 25 202 by the assignee herein). These solutions have the disadvantage, however, that when circumferential speeds of front and rear wheels differ because of tire wear or due to layout necessities, stress within the vehicle occurs at low speed and low load causing not only performance losses but also causing the clamping elements to be held in the clamped position. For this reason further solutions to this problem have been investigated.
The instant invention therefore deals with a free-wheeling coupling comprising the following elements: a first ring which is the driving element in towing mode, a second ring which is the driven element in towing mode, clamping elements which act between the first and the second rings, in both torque flow directions, a cage which holds the clamping elements, and a frictional element acting between the cage and the second ring. DE-A 27 40 638 (U.S. Pat. No. 4,124,085) discloses such a free-wheeling coupling with wedging rollers acting in both directions to transfer drive forces to the second power shaft of a vehicle (in this case, the front axle). A frictional connection exists between the ring pertaining to the second power shaft and the roller cage. When overrunning occurs, a latch mechanism controlled via a second frictional connection to the fixed housing prevents the roller cage from moving into the position in which torque flows from the second power shaft to the drive mechanism of the vehicle or to the first power shaft. Therefore, the second power shaft always runs freely in towing mode, and this affects driving safety in the above-mentioned manner.
In this prior art coupling, the frictional element sensing the absolute rotational speed has the disadvantage of relatively high power losses and is prone to wear. Furthermore, the frictional force of the roller cage cannot be maintained reliably in case of extreme accelerations, e.g., in case of lock-up braking, because of the great inertial forces that are generated. By these, the wedging rollers run up on the ramp surfaces so that the free-wheeling coupling are also locked in thrust mode. However, this also leads to blockage of the wheels of the second driven axle and thereby to the loss of braking stability. It also may lead to the destruction of the free-wheeling coupling.
It is therefore the object of the instant invention to avoid the disadvantages of the two types of constructions mentioned above and to provide a free-wheeling coupling which is dependent on the direction of rotation and which acts as a free-wheeling coupling only during forward travel without any external actuation. The problems discussed at the outset are to be solved by means as simple and inexpensive as possible with minimum wear and without the power loss associated with slip-controlled clutches.